System and method for treating flexible pipes

ABSTRACT

A system and method for treating pipes according to which the flexibility of the pipe is increased or maintained, or at least any decreases in the flexibility of the pipe is minimized, in relatively low temperature environments.

CROSS-REFERENCE

[0001] This application is based on, and claims the priority of, provisional application No. 60/388,664 which was filed by applicant on Jun. 13, 2002.

BACKGROUND

[0002] Flexible pipes for conveying fluid are advantageous in many applications. For example, in systems that transport petroleum-based fluids, the use of flexible pipes reduce life cycle costs as well as the time it takes to construct well lines and place the wells in production.

[0003] However, these types of flexible pipes are not without disadvantages. For example, standard flexible pipes can become relatively stiff, especially at relatively low temperatures. Also, when they are wound and stored on a reel with a relatively small drum diameter, they typically take a quasi-permanent set, which also increases their stiffness. Since the increased stiffness of the pipe makes it difficult to unwind, handle, and install, what is needed are techniques to overcome the above disadvantages by increasing or maintaining the flexibility of the pipe, or at least minimizing the decrease in its flexibility, at relatively low temperature environments.

BRIEF DESCRIPTION OF THE DRAWING

[0004] The drawing is a partial elevational-partial sectional view of a pipe according to an embodiment of the invention.

DETAILED DESCRIPTION

[0005] Referring to the drawing, a pipe 10 is formed by an inner tubular layer 12, surrounded by a tubular insulating layer 14, with the inner surface of the layer 14 being in intimate contact with the outer surface of the layer 12. A layer 16 extends around the layer 14 with the inner surface of the layer 16 being in intimate contact with the outer surface of the layer 14. An outer layer 18 extends around the layer 16 with the inner surface of the layer 18 being in intimate contact with the outer surface of the layer 16.

[0006] It is emphasized that the present application is not limited to the number, composition or construction of the layers 12, 14, 16, and 18. However, in the interest of presenting a non-limiting example, the layer 12 could be in the form of a metallic carcass fabricated from a range of corrosion resistant stainless steel alloys depending on the fluid to be conveyed. Each of the layers 14, 16 and 18 could be fabricated from a plastic material, such as a polymer, so as to be chemically resistant to the fluid being conveyed. Examples of the latter material are high-density polyethylene (HDPE), nylon (PA11), and polyvinylidene fluoride (PVDF).

[0007] Although not shown in the drawing, it is understood that an armor layer can be wrapped around the any one of the layers 12, 14, and 16 to add strength to the pipe 10. This armor layer would be designed to provide resistance to internal and external pressure in the hoop direction, and can take one of several known forms. Also, it us understood that another plastic layer, similar to the layers 14, 16, and 18 could be provided within the layer 12. Other variations in the specific number, composition, and location of the layers are discussed below.

[0008] At any rate, the number, composition, and construction of the layers, such as layers 12, 14, 16, and 18 forming the pipe 10 are such that the pipe 10 is relatively flexible. According to the techniques described below, the pipe 10 will increase or maintain its flexibility, or at least the decrease in its flexibility will be minimized, in relatively low temperature environments.

[0009] According to one technique, rubber modifiers are added to one or more of the layers 12, 14, 16, and 18, such as the layer 16 as shown in the drawing for the purpose of example. The rubber modifiers combat the general increase in the modulus of elasticity of the plastic materials forming the layers 12, 16, and 18 with decreases in temperature, and thus increase, or maintain the flexibility of the pipe 10, or at least minimize the decrease in its flexibility, at relatively low temperature environments. Although the modifiers are shown added to the layer 16 in the drawing, it is understood that they could be added to one or more of the other layers 12, 14, and 18

[0010] Preferably, the rubber modifiers are in the form of thermoplastic elastomers and/or thermoplastic vulcanates and are added to one or more of the layers in a conventional manner in quantities sufficient to reduce the hardness of the layers 12, 14, 16, and 18 to a maximum value of 50. This provides a significant reduction of the bending stiffness of the pipe 10 attributed to relatively low temperature environments, as discussed above.

[0011] The thermoplastic elastomers, also known as thermoplastic elastomeric olefins, can be a blend of two or more polymer systems, each with its own phase. The harder system is a polyolefin, such as polyethylene, polypropylene or polyvinylchloride. The softer system is an elastomer, such as ethylene-propylene rubber or nitrile-butadiene rubber.

[0012] The thermoplastic vulcanates are similar to thermoplastic elastomers, except that the elastomer phase is highly cross linked and finely divided in a continuous matrix of polyolefin. The thermoplastic vulcanates consist of a thermoplastic, and an elastomer, such as butyl rubber.

[0013] It is understood that other combinations of polymers and elastomers may also be added to one or more of the layers 12, 14, 16, and/or 18 to increase or maintain the flexibility of the pipe 10, or at least to minimize the decrease in its flexibility, at relatively low temperature environments.

[0014] According to another technique to increase, or maintain the flexibility of the pipe 10, or at least to minimize the decrease in its flexibility, at relatively low temperature environments, the effective thickness of one or more of the pipe layers 12, 14, 16, and 18 is minimized. This reduction in thickness results in a reduction in the outer diameters of the layers and the overall diameter of the pipe, which effects a relatively large reduction in the stiffness of the pipe. To this end, the stiffness is approximated by summing the stiffness of the individual layers. The stiffness of any given layer is governed by the following formula:

Stiffness=EI

[0015] where

[0016] E is the elastic modulus of the material, and

[0017] I is the moment of inertia of the cross sectional area.

[0018] The moment of inertia of a pipe layer is defined by the following formula: $I = {\frac{\pi}{64}\left( {D_{o}^{4} - D_{i}^{4}} \right)}$

[0019] where

[0020] D_(o)=outside diameter of the layer and

[0021] D_(i)=inside diameter of the layer

[0022] The average thickness of a layer is defined by:

T=D _(o) −D _(i)

[0023] It can be seen that reducing the thickness of one or more of the layers 12, 14, 16, and 18 will result in a large reduction in the stiffness of the pipe. For example, reducing the outer sheath thickness from 6 mm to 5 mm on a pipe 10 with an outside diameter of 200 mm, will reduce the bending stiffness of that layer by approximately 18%. In this context, the drawing is not drawn to scale and, although the respective thicknesses of the layers 12, 14, 16, and 18 are shown in the drawings as being substantially similar, it is understood that, in accordance with the above, the thickness of one or more of the layers would be less than shown in the drawing.

[0024] According to another technique for increasing or maintaining the flexibility of the pipe 10, or at least minimizing the decrease in its flexibility, at relatively low temperature environments, one or more of the layers 12, 14, 16, or 18 is formed by tape. The use of tape layers, especially if there is no adhesive applied to the tape, aids in reducing the stiffness of the pipe since the tape layers are generally thinner than solid extruded layers and thus reduce the outer diameter of the pipe and the stiffness of the pipe, for the reasons described above. Also, if there is no adhesive on the tape, the adjoining wraps of tapes are able to slide relative to each other to accommodate the effective strain imposed by bending the pipe, as will be discussed below. Although the respective thicknesses of the layers 12, 14, 16, and 18 are shown in the drawings as being substantially similar, it is understood that if one or more of the layers is formed by tape, its thickness relative to the other layers would be less than that shown in the drawing.

[0025] If the pipe is stored on a storage reel having a relatively small-diameter drum in a relatively low temperature environment, such as outdoors at minus 40° C. or less, the polymeric layers, 14, 16 and 18 will assume the relatively high curvature of the drum through a behavior called stress relaxation and thus become relatively tightly coiled. Once this condition is reached, the pipe will tend to remain in the tightly coiled configuration for an extended period which, of course, increases its stiffness. Therefore, according to another technique for increasing or maintaining the flexibility of the pipe 10, or at least minimizing the decrease in its flexibility under these conditions, the pipe is stored in a more moderate temperature environment and/or on storage reels having relatively large-diameter drums.

[0026] In particular, the environmental temperature should be no less than approximately 5° C. (which can vary based on wind conditions, humidity, etc.), and the diameter of the drum of the storage reel is such that the maximum strain due to bending (as calculated according to the method described below) is preferably no greater than five percent when compared to the pipe when it is not bent. If either one of these conditions are met, the increase in stiffness that would otherwise occur is reduced, and, of course, if both conditions are met the increase in stiffness is reduced even further.

[0027] In the context of the environmental temperature, the pipe 10 can simply be stored in an environment that enables its temperature to be no less than 5° C., or if this is practicable, the pipe can be heated, such as by circulating relatively warm water or warm air through the pipe, to bring its temperature up to this value. Therefore, since the pipe 10 is fabricated, at least in part by a plastic material, preferably in the form of a polymer, the increase in temperature of the pipe will cause a corresponding increase the elastic modulus.

[0028] In the context of reducing the strain on the pipe by winding it onto a relatively large drum of a storage reel, as discussed above, the strain at the outer layers of the pipe 10 is calculated according to the following formula:

ε=d/D

[0029] where

[0030] ε=the strain at the outer fiber of the material, equal to the elongation per of unit length at the outer fiber of the layer.

[0031] d=the outer diameter of the pipe, and

[0032] D=the diameter of the centerline of the pipe in the storage condition.

[0033] The mean diameter of the pipe 10 at the inner wrap on a drum of a storage reel is equal to the drum diameter plus the pipe outside diameter. The strain changes in the pipe are inversely proportional to the drum diameter.

[0034] It is understood that two or more of the techniques discussed above can be combined. For example, the pipe 10, when in the relatively cold environment discussed above, can initially be heated, or placed in a relatively warm environment, as discussed above to raise its temperature to the values described above; after which it can be transferred to a storage reel having a relatively large diameter that insures that the strain discussed above does not exceed five percent. Still other techniques disclosed above can be combined.

[0035] It can also be appreciated that when the pipe 10 is stored in a straight or reverse bent condition in a relatively cold environment, if it is prepared for service by unbending it too rapidly at the relatively low temperature, the polymeric layers 14, 16, and/or 18 may fail due to tensile overload of the outer layers or buckling of the inner layers. Therefore, in accordance with the above, the pipe 10 is heated to at least 5° C., and\or is moved to a reel having a relatively large-diameter drum in accordance with the above. In addition to increasing or maintaining the flexibility of the pipe 10, or at least minimizing the decrease in its flexibility, the above techniques will reduce the chance of failure of the polymeric layers 14, 16, and 18.

[0036] Thus according to each of the above techniques the flexibility of the pipe 10 is increased or maintained, or at least any decreases in its flexibility is minimized, at relatively low temperature environments, resulting in a pipe that is relatively easy to unwind, handle, and install.

Variations

[0037] 1. The composition of each of the layers disclosed above can be varied within the scope of the invention.

[0038] 2. One or more of the layers discussed above can be eliminated.

[0039] 3. One or more of the layers discussed above can be replaced by another layer of a different design.

[0040] 4. Two or more versions of each layer discussed above can be provided.

[0041] 5. Additional layers of a different design, can be provided under, over, or between, the layers discussed above.

[0042] 6. The relative thicknesses of the layers discussed above are shown in the drawing only for the purpose of example, it being understood that these relative thicknesses can be varied within the scope of the invention. For example, one of the layers can be in the form of tape as discussed above, in which case the layer would be much thinner than depicted in the drawing.

[0043] 7. The relative radial positions of the layers discussed above can be changed.

[0044] 8. The adjacent windings of the strip forming the layer 12 do not have to be interlocked.

[0045] 9. The spatial references, such as “under”, “over”, “between”, “outer”, “inner”, “around”, and “surrounding” are for the purpose of illustration only and do not limit the specific orientation or location of the layers described above.

[0046] Although only a few exemplary embodiments of this invention have been described in detail above, those skilled in the art will readily appreciate that many other modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. 

What is claimed is:
 1. A method of treating a flexible pipe to prevent or minimize increase in its stiffness in relatively low temperature environments, the method comprising adding rubber modifiers to the material forming the pipe to prevent or minimize any increase in the modulus of elasticity of the plastic material due to the low temperature.
 2. The method of claim 1 wherein the pipe is formed by at least one layer of plastic material.
 3. The method of claim 1 wherein the pipe is formed by two or more layers of plastic material and wherein the rubber modifiers are added to at least one of the layers.
 4. The method of claim 1 wherein the rubber modifiers are added in sufficient quantities to reduce the hardness of the pipe to a maximum value of
 50. 5. The method of claim 1 wherein the rubber modifiers are in the form of thermoplastic elastomers.
 6. The method of claim 5 wherein the thermoplastic elastomers are a blend of two or more polymer systems, each with its own phase.
 7. The method of claim 6 wherein one of the polymer systems is a polyolefin.
 8. The method of claim 7 wherein the polyolefin is selected from the group consisting of polyethylene, polypropylene, and polyvinylchloride.
 9. The method of claim 6 wherein one of the polymer systems is an elastomer.
 10. The method of claim 9 wherein the elastomer is selected from the group consisting of ethylene-propylene rubber and nitrile-butadiene rubber.
 11. The method of claim 1 wherein the modifiers are in the form of thermoplastic vulcanates.
 12. The method of claim 11 wherein the modifiers comprise thermoplastic elastomers and thermoplastic vulcanates.
 13. The method of claim 1 further comprising storing the pipe in an environment in which the temperature is no less than approximately 5° C. to reduce the stiffness of the pipe when compared to a pipe that is stored in an environment greater than 5° C.
 14. The method of claim 1 further comprising heating the pipe to a temperature of no less than approximately 5° C. to reduce the stiffness of the pipe when compared to a pipe that is at a temperature less than 5° C.
 15. The method of claim 14 further comprising the step of storing the heated pipe on a storage drum, the diameter of which is such that the maximum strain due to the bending of the pipe on the drum is no greater than five percent when compared to the pipe when it is not bent.
 16. The method of claim 1 further comprising the step of storing the pipe on a storage drum, the diameter of which is such that the maximum strain due to the bending of the pipe on the drum is no greater than five percent when compared to the pipe when it is not bent.
 17. A method of treating a flexible pipe to prevent or minimize increase in its stiffness in relatively low temperature environments, the method comprising storing the pipe in an environment in which the temperature is no less than approximately 5° C. to reduce the stiffness of the pipe when compared to a pipe that is stored in an environment greater than 5° C.
 18. The method of claim 17 wherein the pipe is formed, at least in part, by a plastic material, and further comprising the step of adding rubber modifiers to the material forming the pipe to prevent or minimize any increase in the modulus of elasticity of the plastic material due to the low temperature.
 19. The method of claim 17 further comprising storing the pipe on a storage drum in the environment, the diameter of the drum being such that the maximum strain due to the bending of the pipe on the drum is no greater than five percent when compared to the pipe if it is not bent.
 20. A method of treating a flexible pipe to prevent or minimize increase in its stiffness in relatively low temperature environments, the method comprising heating the pipe to a temperature of no less than approximately 5° C. to reduce the stiffness of the pipe when compared to a pipe that is at a temperature less than 5° C.
 21. The method of claim 20 wherein the step of heating comprises circulating relatively warm water or relatively warm air through the pipe.
 22. The method of claim 20 wherein the pipe is formed, at least in part, by a plastic material, and further comprising adding rubber modifiers to the plastic material to prevent or minimize any increase in the modulus of elasticity of the plastic material due to the low temperature.
 23. The method of claim 20 further comprising storing the heated pipe a storage drum, the diameter of which is such that the maximum strain due to the bending of the pipe on the drum is no greater than five percent when compared to the pipe if it is not bent.
 24. A method of treating a flexible pipe to prevent or minimize increase in its stiffness in relatively low temperature environments, the method comprising the step of storing the pipe on a storage drum, the diameter of which is such that the maximum strain due to the bending of the pipe on the drum is no greater than five percent when compared to the pipe if it is not bent.
 25. The method of claim 24 wherein the step of storing eliminates any stress relaxation that would be caused if the diameter of the storage drum causes a maximum strain in excess of five percent.
 26. The method of claim 24 wherein the strain is calculated according to the following formula: εd/D where ε=the strain at the outer fiber of the material, equal to the elongation per of unit length at the outer fiber of the layer d=the outer diameter of the pipe, and D=the diameter of the centerline of the pipe in the storage condition.
 27. The method of claim 24 wherein the pipe is formed, at least in part, by a plastic material, and further comprising adding rubber modifiers to the material to prevent or minimize any increase in the modulus of elasticity of the plastic due to the low temperature.
 28. The method of claim 24 further comprising storing the pipe in an, environment in which the temperature is no less than approximately 5° C. to reduce the stiffness of the pipe when compared to a pipe that is stored in an environment greater than 5° C.
 29. The method of claim 28 wherein the step of storing occurs after the temperature of the pipe is at least 5° C.
 30. The method of claim 24 further comprising heating the pipe to a temperature of no less than approximately 5° C. to reduce the stiffness of the pipe when compared to a pipe that is at a temperature less than 5° C.
 31. The method of claim 30 wherein the step of heating is before the step of storing. 